Police have been ordered to review their custody policy after a murder suspect died within hours of being freed from questioning in Wellington.

An Independent Police Conduct Authority report into police actions before the death of Timothy Parlane, 27, of Lower Hutt, was completed last year, but its full findings remain secret after the authority decided not to publish them.

Next month will mark two years since Matthew Hall, 35, was found stabbed to death in his flat in Broderick St, Johnsonville. Police have not charged anyone with his murder and have now handed the case over to Wellington Coroner Ian Smith.

In April 2011, The Dominion Post reported that Mr Parlane had confessed to a woman he was dating that he had murdered Mr Hall after the pair fell out. Mr Parlane was questioned by police on March 5, and died hours later when he was hit by a train.

The woman said she had warned police that Mr Parlane had threatened suicide if they were to interview him.

The findings of the IPCA investigation were sent to Police Commissioner Peter Marshall and Mr Parlane's family in August last year.

Police accepted its recommendations, which the IPCA ruled would not be made public.

The authority was asked to reconsider that decision, given that the findings had prompted a policy review. An IPCA spokeswoman said: "I can only confirm that a number of factors were taken into account regarding the decision not to release that report publicly. Those circumstances haven't changed and the authority won't be revisiting that decision."

Police also declined to release the report. A spokeswoman said: "Police have accepted the IPCA's recommendation that the commissioner reviews current policy, modifies it if necessary and develops an appropriate custodial management training programme. Work is now under way to review the current policy framework."

The IPCA receives about 2000 complaints each year. In 2010-2011, it produced 17 public reports. The police investigation into who killed Mr Hall was completed late last year and has been reviewed by a senior officer, the Wellington district commander and the police legal section.

The officer in charge of the investigation, Detective Senior Sergeant Dave Thornton, would not answer whether the case was being treated as unsolved or resolved, saying only: "As this file is now before the coroner, it is not appropriate for us to discuss the actual outcome of our investigation.

"Whilst no arrest has been made in relation to Matthew Hall's death, police are confident our investigation was a thorough one and all possible lines of inquiry were exhausted."

He would not confirm whether Mr Parlane was the sole suspect, but said police did not believe there was any risk to the public.

In an interview last year, Mr Thornton said it had been difficult to corroborate what Mr Parlane had said without being able to speak to him. The confessions he had given to two separate women differed from one another, he said. "And some of the things he's described at the scene . . . some of it is consistent, some of it isn't consistent."

The long-running inquiry involved the FBI and the Australian Federal Police testing hundreds of forensic samples. Police also contacted the United States Government, which went to court and had a warrant issued against Facebook to release the personal pages of suspects in the case.

The deaths of Mr Hall and Mr Parlane are now being investigated by Mr Smith. He is yet to decide if and when an inquest or inquests will be held.

THE STORY SO FAR

February 2011: Matthew Hall is found stabbed to death in his bed in Johnsonville, Wellington.

March 2011: Lower Hutt man Timothy Parlane is interviewed by police in connection with Mr Hall's death and released soon after. Hours later he is hit by a train. An IPCA investigation is started.

April 2011: The Dominion Post reveals Mr Parlane confessed to a woman he was dating that he had murdered Mr Hall.

August 2012: The IPCA investigation into police conduct before Mr Parlane's death is completed, but it decides not to publish its findings.

December 2012: The police file is passed on to the Wellington coroner.